The invention pertains to a sensor device for detecting a physical measurement variable including a sensor module as well as an electronic circuit module, wherein each value of the measurement variable is determined by comparison of an electrical voltage signal generated in the sensor module with a reference voltage generated in the circuit module and each value is output as a digital or analog coded signal at an output.
More specifically, the invention pertains to a magnetostrictive length measurement device that is described, e.g., in U.S. Pat. No. 5,334,933. This length measurement device essentially consists of an elongated, acoustic delay line containing a ferromagnetic material exhibiting a magnetostrictive effect, a sensor head at one end of this line that contains the measurement electronics and the evaluation electronics, and a permanent magnet that can be shifted along the line.
The length measurement is achieved by determining the position of the permanent magnet along the delay line relative to the sensor head.
In this way an electrical current pulse is induced in this delay line and simultaneously a time measurement is started. The magnetic field caused by this current pulse interacts with the magnetic field at the location of the permanent magnet in such a way that at that point a voltage state in the delay line is generated in the form of an acoustic torsional pulse that is transmitted along the delay line at the speed of sound that is typical for the material. The acoustic torsional pulse is converted into an equivalent electrical signal in the sensor head and when it exceeds a reference value, an output signal is generated that ends the time measurement.
The determined time is proportional to the length to be measured so that the desired measurement result can be output in digital or analog coded form by the electronics of the sensor head.
Since the amplitude of the torsional pulse and thus also the electrical signal generated in the sensor head by means of this pulse becomes smaller with increased measured length, it is necessary to adjust the threshold value of the comparator, e.g., as described in the cited patent.
However, the amplitude of the electrical signal is also subject to other influencing factors, wherein the ambient temperature plays an important role, so that for the evaluation of the tolerance position of the total system, particularly in view of a sufficient signal to noise margin, the knowledge of the absolute amplitude of the electrical signal is required. However, access to this information with sensor devices of the type used at the present time is not possible.
The task of the present invention is to refine the sensor device of the described art such that information about the amplitude of the internal electrical signal is accessible for the evaluation of the current tolerance situation at all times without requiring an additional interface.
This is achieved in that the amplitude of this electrical signal is represented by the period of the delay between the activation of the supply voltage for the sensor device and the output of the first valid measurement value.
In addition, the task of the present invention is to provide a method for use that can execute simple and fast function testing of such a sensor device.
This is achieved in that the amplitude of the electrical voltage signal generated in the sensor module that is determined by the period of the delay is used for the evaluation of the tolerance situation of the sensor device as well as the output of a good-bad decision in particular.